Coatings based on epoxy resins are important industrial products. The largest volume of these products is used for the protection and decoration of large metal or concrete structures such as bridges, ships, industrial tanks, etc., where application of the coating must be performed under ambient conditions. Epoxy coatings of this type have proven themselves to offer an excellent combination of corrosion resistance, water resistance, abrasion resistance, solvent resistance and other desirable coatings properties, and do so in a cost effective manner.
Most epoxy resin coatings designed for ambient application employ polyfunctional amines as the curing agent, either alone or in some cases in combination with other curing agents. Several classes of amine curing agents are used commercially, including aliphatic amines, amine adducts, Mannich bases, polyamides, and polyamidoamines which are also known as amidoamines. They are described more fully in W. R. Ashcroft, Curing Agents for Epoxy Resins, in B. Ellis (ed.), Chemistry and Technology of Epoxy Resins, Blackie Academic and Professional, London, 1993, pp 37-71.
Among these curing agents, polyamides are a particularly important class of curing agent for the formulation of coatings. Polyamides are comprised of the reaction products of dimerized fatty acid (dimer acid) and polyethyleneamines, and usually a certain amount of monomeric fatty acid which helps to control molecular weight and viscosity. "Dimerized" or "dimer" or "polymerized" fatty acid refers, in a general way, to polymerized acids obtained from unsaturated fatty acids. They are described more fully in T. E. Breuer, `Dimer Acids`, in J. I. Kroschwitz (ed.), Kirk-Othmer Encyclopedia of Chemical Technology, 4.sup.th Ed., Wiley, N.Y., 1993, Vol. 8, pp. 223-237. Dimer acid is usually prepared by the acid catalyzed oligomerization under pressure of certain monomeric fatty acids, usually tall oil fatty acid (TOFA), though sometimes other vegetable acids are substituted. Commercial products generally consist of mostly (&gt;70%) dimeric species, with the rest consisting mostly of trimers and higher oligomers, along with small amounts (generally less than 5%) of monomeric fatty acids. Any of the higher polyethyleneamines can be employed in the preparation of polyamides, such as diethylenetriamine (DETA), triethylenetetramine (TETA), tetraethylenepentamine (TEPA), or pentaethylenehexamine (PEHA), though in actual commercial practice the polyethyleneamine most commonly employed is TETA.
Polyamidoamines are another important class of curing agents. Whereas commercial polyamides are available in viscosities ranging from about 10,000 to 500,000 cP, polyamidoamine viscosities are generally in the range of about 100 to 700 cP. This is clearly an advantage over polyamides when trying to formulate coatings and related products with no solvent, or a minimum amount of solvent. They are the reaction product of a monomeric fatty acid such as TOFA and a polyethyleneamine. The polyethyleneamine most commonly employed in this case is TEPA, even though TETA is a less expensive raw material. The reason that TEPA is used for polyamidoamines is that it yields products that remain completely liquid at temperatures normally encountered in their use and storage. Polyamidoamines based on TETA, on the other hand, generally have a marked tendency to crystallize even at room temperature. This creates considerable difficulties for end users, who must melt the material to obtain a uniform mixture before use. Alternatively, the product could be supplied in a solvent, but this is a disadvantage because environmental regulations now restrict the amount of solvent allowed in formulations, and many epoxy applications require the use of formulations containing no solvent.
Polyamides are employed because they allow for the formulation of coatings with an excellent combination of water and corrosion resistance, most likely due to the hydrophobicity imparted by the fatty nature of the starting materials. They also can offer excellent flexibility and reasonable cure speeds (drying times). Polyamidoamines are used much less in coatings than are polyamides, though they also yield hydrophobic films with good water resistance. Part of the reason is that polyamidoamines generally yield less flexible films than polyamides. However, probably more important is the poor surface appearance that is obtained with the use of polyamidoamines when compared to polyamides.
There is a tendency for many epoxy curing agents to rise to the surface of a coating during the cure. This can leave a greasy film on the surface of the coating known as exudate, which detracts from the appearance, and which can also lead to intercoat adhesion failure if the epoxy is a primer or mid-coat. Under adverse application conditions such as high humidity, a high concentration of the amines at the surface can result in the formation of whitish precipitates on the surface which are probably bicarbonate and/or carbamate salts, a problem known in the industry as blush. Polyamides are much better than polyamidoamines in this regard. Though they may not be completely free of blush and exudate if applied immediately after mixing, it is generally found that if the materials are allowed to react for about 30 minutes to 1 hour before application that the blush and exudate are eliminated. This waiting period is called an induction time.
Polyamidoamines, on the other hand, generally give much higher levels of blush and exudate, and even after several hours of induction they may still exhibit blush and exudate. Polyamidoamines also display a strong tendency to `crawl` and `puddle` on the substrate, and generally yield films that have unacceptable appearance. Flow additives added to such a formulation are generally less than satisfactory in curing this problem. Once again, polyamides are superior to polyamidoamines in this property.
Another important property of an epoxy based coating is the speed at which it cures, or its `dry time`. Generally, users of epoxy coatings prefer dry times to be as short as possible for obvious reasons.
Another important property of an epoxy based coating is the mixed viscosity of the epoxy resin and the curing agent. This will clearly have an effect on how much solvent is required to reduce the viscosity of a coating formulation to the point where acceptable application properties are achieved. Usually curing agents are employed at stoichiometric ratios of equivalents of epoxide to equivalents of amine hydrogen close to unity. This stoichiometry generally yields the highest crosslink density final product, and most properties are also maximized at this ratio. Polyamidoamines tend to have amine hydrogen equivalent weights from about 65 to 105. If equivalent weight were higher, curing agent would be a higher percentage of the total amount of material in the epoxy resin and curing agent blend at 1:1 stoichiometry. If the curing agent is significantly lower in viscosity than epoxy resin then increasing the equivalent weight of the curing agent will result in a lower mixed viscosity.
GB 789,108 discloses polyamides made by condensation of a polyamine with a saturated aliphatic acid of the formula HOOC(CH2)nCOOH where n is an integer from 3-12 or an aromatic dicarboxylic acid. Aromatic dicarboxylic acids include isophthalic and terephthalic acid. It is claimed that the solubility and compatibility properties of the polyamides may be improved if up to 60 mole % of the dicarboxylic acid is replaced by a monocarboxylic acid. Suitable monocarboxylic acids include fatty acids and benzoic acid, and it is "understood that in the formation of the polyamides mixtures of polyamines and/or carboxylic acids may be used" (p.2, col.1, I. 18-19.) One example shows a mixture of adipic acid and benzoic acid reacted with TETA, and another shows dimethyl terephthalate reacted with TETA.
U.S. Pat. No. 4,732,966 discloses polyamides containing free amino groups derived from (a) a polybasic acid component comprising polymeric fatty acid (i.e., dimer fatty acid) together with from 10 to 90 mole %, based on the total acid component, of one or more aromatic dicarboxylic acids, and (b) a polyamine containing at least 50 mole % of one or more aliphatic polyamines containing at least three amino groups per molecule. In the disclosure, it is stated that "the acid component should contain at least 10 mole % of polymeric fatty acid, preferably at least 30 mole % thereof" (Col 2/51-53).
U.S. Pat. No. 4,086,197 discloses polyamino-amide hardener prepared from 90 to 60 parts of at least one unsaturated monomeric fatty acid having at least 12 carbons, 10 to 40 parts of an acid having a molecular weight less than 200, 20 to 80 parts of a polyalkylene-polyamine, and 80 to 20 parts of isophorone diamine, with the ratio of acid to amine components being 1 to 2 eq. of acid per mole of amine. Benzoic acid is among the monocarboxylic acids with a molecular weight less than 200 that is recommended and also was used in the examples. I. Slowikowska, A. Kozlowski, and K. Zawalska, Ipolimery-Tworzywa Wielkoczasteczkowe, 1980, pp. 291-292 investigated the cure characteristics and mechanical properties of epoxy resins cured with polyamidoamines based on benzoic acid and TETA.
R. H. E Munn, Journal Oil and Coulor Chem. Assoc., 1987 (10), pp 300-304 describes the modification of traditional dimer acid-based polyamide curing agents with terephthalic, and particularly isophthalic acid. Such modification leads to good compatibility and dry speed and freedom from surface defects.